Oxidation of ammonia using chlorine may be used to remove low levels of ammonia from water, for example wastewater. Chlorine gas rapidly disproportionates in water to form hypochlorous acid (HOCl), hydronium ion (H3O+), and chloride ion (Cl−) according to the following reaction:2H2O(I)+Cl2(gas)HOCl+H3O++Cl−
The hypochlorous acid is a weak acid with a pKa of 7.6 at 25° C. and can transfer a proton with formation of hypochlorite ion (OCl—).
The hypochlorous acid reacts with ammonia according to the following reactions:2NH3+3HOCl→N2(gas)+3H++3Cl−+3H2Owhich means that one mole of ammonia reacts with 1.5 moles of chlorine or, in terms of weight 1 mg of ammonia as N needs to react with 7.6 mg of chlorine as Cl2 in order to produce nitrogen gas. If a smaller ratio is used inorganic chloramines are produced according to the following reactions:NH3+HOCl→NH2Cl+H2ONH2Cl+HOCl→NHCl2+H2ONHCl2+HOCl→NCl3+H2OThe mixture of monochloramine, dichloramine and trichloramine has different proportion depending on pH, temperature, and ratio of chlorine to ammonia (Cl2:N).
The main additional oxidized product may be nitrate (NO3−) formed by following reaction:NH4++4HOCl→NO3−+4Cl−+6H++H2O
It is desirable reduce the amount of hypochlorous acid and/or hypochlorite, generated by the reaction between chlorine gas and water, remaining in the waste water after the ammonia has been oxidized.